Danny Kirwan in 1968. His compositions would help to move Fleetwood Mac away from their strictly blues roots towards the more melodic soft-rock that made them famous.
Photograph: Alamy

Although he was only 18 when he joined Fleetwood Mac in 1968, Danny Kirwan, who has died aged 68, rapidly became a significant creative force within the group in their early years. It was the guitarist Peter Green who achieved enduring “guitar hero” status with the band, but Kirwan was also a fluent and accomplished player with a delicate touch, his playing particularly recognisable for its use of vibrato.

He was also a prolific songwriter whose compositions would help to move Fleetwood Mac away from their strictly blues roots towards the more melodic soft-rock that turned them into one of the world’s most successful acts.

Kirwan had been in the group for two months when he made his first recording with them, playing on their Green-composed single Albatross, a lilting instrumental assembled from contrasting guitar parts. It was an auspicious beginning, since this would be the band’s only UK No 1 hit. His first album with them, Then Play On (1969), contained seven of his songs, including the string-accompanied ballad When You Say among more conventionally bluesy material.

Danny Kirwan, left, with other members of Fleetwood Mac in 1969. From left: Mick Fleetwood, Jeremy Spencer, John McVie and Peter Green. Photograph: Rex/Shutterstock

He had more writing credits on Kiln House (1970) – the group’s first album after the departure of Green – including the bouncy rocker Tell Me All the Things You Do, and he wrote the single Dragonfly (1970), with lyrics from a poem by WH Davies. Green considered Dragonfly to be the best song Kirwan ever wrote.

Future Games (1971) included the Kirwan-penned opening track Woman of 1000 Years, a piece of dreamy California-style psychedelia, and his proto-country rock effort Sometimes. Bare Trees (1972), the last Mac album Kirwan appeared on, featured five more of his songs, including the almost Eagles-like Child of Mine and the poignant soft-rock of Dust (the latter taking its lyrics from Rupert Brooke’s poem of the same name).

Kirwan can thus be seen as the missing link between the original Fleetwood Mac, planted squarely in the British blues boom, and the band’s megastar LA-based incarnation featuring Stevie Nicks and Lindsey Buckingham , when it would sell 40m copies of its 1977 album Rumours.

But Kirwan was unable to cash in on the band’s subsequent commercial bonanza. He had always been emotionally fragile, and Green recounted that Kirwan would often be in tears while he was playing. The strain of touring and performing drove him to drink and drugs, and he often neglected food altogether. He finally quit during a US tour in 1972, when he flew into a rage in the dressing room before one of the shows, smashed his Les Paul guitar and refused to take the stage with the rest of the band. Afterwards Mick Fleetwood told Kirwan he was out of the band.

Kirwan was born in Brixton, south London, though obscurity surrounds his upbringing. At 17 he was playing in a three-piece band called Boilerhouse, and after he persuaded Fleetwood Mac’s producer Mike Vernon to come and see them, Vernon recommended them to Green, who invited Boilerhouse to be the support band at Fleetwood Mac shows. Green had not been happy with his co-guitarist Jeremy Spencer and was looking for another guitar player, so Kirwan was invited aboard, joining the lineup in August 1968.

“I was lucky to have played for the band at all,” Kirwan told the Independent in a rare interview in 1993, after he had stepped out of the limelight. “I did it for about four years, to about 1972, but I couldn’t handle the lifestyle and the women and the travelling.” At this time he had been living in a St Mungo’s homeless hostel in central London, but had been tracked down by Fleetwood, who had last seen him in 1980.

After leaving Fleetwood Mac, Kirwan had put in a blink-and-you-missed-it stint with a band called Hungry Fighter, who played one solitary gig and made no recordings. He made three solo albums on the DJM label in the 1970s, Second Chapter (1975), Midnight in San Juan (1976) and Hello There Big Boy! (1979), but though the music was often melodic and attractive, Kirwan’s absence from live performance and lack of public visibility meant that the discs sold miserably and failed to chart.

He subsequently drifted away from music altogether, spending 10 years living rough and in a basement flat in Brixton, surviving on social security and royalty payments from his Fleetwood Mac work. In 1998, he was inducted into the Rock and Roll Hall of Fame with Fleetwood Mac, but did not attend the ceremony.

He is survived by a son, Dominic, from his marriage to Clare Morris, which ended in divorce.

• Daniel David Kirwan, guitarist, singer and songwriter, born 13 May 1950; died 8 June 2018